1. Technical Field
This invention relates to an improved radio receiving apparatus, and more particularly, to a mobile electronic device which includes headphones coupled by a cable to a receiving unit, wherein the cable is adapted to supply audio signals from the receiving unit to the headphones, and also to pick up and couple radio signals to the receiving unit.
2. Discussion of Related Art
The state of the art is provided by document U.S. Pat. No. 4,369,521. This document discloses an electric circuit to filter a radio signal from a ground line of a stereo headphone. The described circuit uses an inductor or a coil to block the radio frequencies (RF) from the output of the audio amplifiers and from ground, and a capacitor to block the audio frequencies from the radio frequency (RF) receiver circuit. Basically, U.S. Pat. No. 4,369,521 discloses a radio apparatus that is to be operated with headphones and that uses a frequency splitter with a single cable to provide an antenna and a headphone cable, for filtering out the RF signal from the used headphone cable signal ground. This solution is independent from the used cable type of the headphone. With the use of this document it is possible to use different headphones with different cable types e.g. cables with shielding from the signal wires (coaxial cables). Another advantage of this solution is that it is possible to use different headphones independent from the used cable type.
Other efforts to use e.g. a headphone cable as an antenna are known with balanced headphone cables. In some of these cases the signal is coupled out via capacitors from each of the four conductors of the balanced stereo output, because there is no common ground signal.
The two above approaches for using a headphone cable have the common feature that they have much RF loss, and further there is waste energy in the resistance of the inductor. Additionally this solution is not suitable for use with mobile terminal devices. Therefore, a new antenna concept is needed to overcome these problems.